Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received January 1, 2015; final manuscript received April 5, 2015; published online June 2, 2015. Assoc. Editor: Mohammed Daqaq.

Abstract

In the field of nondestructive testing (NDT), a suitable defect identification parameter plays an important role in evaluating the reliability of structures or materials. In this work, we proposed a NDT method which detects the sample's local contact stiffness (LCS) based on the contact resonance of a piezoelectric cantilever. First, through finite element analysis (FEA) we showed that LCS is quite sensitive to typical defects including debonding, voids, cracks, and inclusions, indicating that LCS could be a good identification parameter. Then, a homemade NDT system containing a piezoelectric cantilever was assembled to detect the sample's LCS by tracking the contact resonance frequency (CRF) of the cantilever-sample system. Testing results indicated that the proposed NDT method could detect the above mentioned defects efficiently and precisely. The cantilever-stiffness dependent detection sensitivity was specially investigated and the stiffer cantilevers were found to be more sensitive to small defects, while the softer cantilevers were more suitable for large defects detecting with smaller pressing force. Finally, the detection limit of this NDT method is investigated both experimentally and computationally. The proposed LCS-based NDT method could be very promising for defect detecting in noncontinuous structures and composite materials.

The calculated LCS variations across different defects using FEA models: (a) LCS variations across two debonding defects with the same depth of 5 mm; (b) LCS variations across a void and a stiff inclusion with the same depth of 20 mm, the diameters of both defects are 10 mm; (c) LCS variations across void defects with different sizes and depths; and (d) LCS variations across vertical cracks with the same length of 10 mm and depth of 15 mm, but different COD

Illustration of the piezoelectric unimorph cantilever for LCS detection: (a) basic assembly and (b) simplified mechanical model for contact vibration of the cantilever-sample system. The contact interaction between the cantilever tip and the sample is approximated by a linear spring with the stiffness of k*.

The two-layer polymer/PMMA specimen with a prefabricated debonding defect. (a) Diagram of the specimen. (b) Photography of the specimen (top view). The scanning area is denoted by the square with the size of 40 mm × 40 mm in which the debonding defect can be clearly seen.

The aramid fiber composite specimen with a longitudinal crack on the bottom surface. (a) Diagram of the specimen. The thickness of the specimen is 3 mm, and the crack runs through the specimen with a width of 4 mm and an averaged depth of 1 mm. (b) Photography of the specimen. The scanning area is denoted by the square with the size of 20 mm × 20 mm.

Imaging results to determine the debonding detection limit of this LCS-based NDT system. (a) Photography of the two-layer specimen with triangle-shaped debonding, the acute angle for the triangle is 3 deg. The scanning area is denoted by the red rectangle with the size of 10 mm × 40 mm. (b) Amplitude image by the SF mode with the pixel of 20 × 80. (c) LCS image by the RT mode with the pixel of 15 × 60.

Return to: Local Contact Stiffness Detection for Nondestructive Testing Based on the Contact Resonance of a Piezoelectric Cantilever

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